Convert a tab into a popup window works well. but when I merge popup, the firefox windows missed. I use "Sizer" software to get windows current position, it is outside screen. (my monitor resolution is 2560x1440, but firefox windows position at left:2757 top:83)

I use a trio of extensions for pop-ups.

Focus Popup https://addons.mozilla.org/addon/focus-popup/ has a keyboard shortcut (Control-Shift-7) and can duplicate a tab into a pop-up (without history) whilst leaving the original (with history).

PopUp https://addons.mozilla.org/addon/popup/ preserves tab history when popping up, and allows me to choose the window in which a pop-up should be restored, but has no keyboard shortcut – https://github.com/aqt/webext-popup/issues/37

Popup Tab https://addons.mozilla.org/addon/popup-tab/ preserves tab history when popping up, and has a keyboard shortcut (Control-Shift-8) that works with Waterfox 56.2.7.1.

I hope this add-on has added the ability to automatically turn pop-up into tab.

Currently this plugin is only able to merge back popups that are popup by this plugin.